The glomerular filtrate is usually acidified by renal tubules and collective ducts. The pH of urine normally ranges from 4.5 to 7.5. Urine pH should be measured with a pH meter on a freshly voided urine sample as on standing diffusional loss of CO2 or bacterial contamination may occur changing the pH value. Urine pH is important for diagnosis of RTA.

Specific gravity determines the ability of the kidney to concentrate or dilute urine. It is precisely measured with the help of a refractometer. Normal specific gravity of a urine ranges from 1.001 to 1.030. It is necessary in diagnosis of renal or nephrogenic Diabetes Insipidus and Psychogenic DI.

Normally, Osmolality of urine is between 40 to 1200mosm/kg.
Osmolality = ( Specific gravity -1.000) x 40,000.

Fresh-centrifuged sample should be examined microscopically for presence of WBCs & RBCs. Centrifuged sediment should be examined to identify crystals, casts, and squamous cells. If microscopy is delayed, the specimen should be refrigerated at + 40C to prevent bacterial overgrowth and dissolution of cells and casts.

Red Blood Cells : >5RBC/cu mm in a HPF in 10ml of centrifuged sample is considered abnormal. Once hematuria is established, it is important to determine the source of bleeding. RBC casts, dysmorphic crenated RBCs suggest glomerular bleed whereas normal RBC morphology suggests lower urinary tract bleed.

White Blood Cells : > 10 WBC/cu mm for body and > 50 WBC/cu mm for girls is abnormal and suggestive of pyuria that is seen in UTI or with acute glomerulonephritis.

Epithelial Cells : They arise from renal tubules and transitional cells arise from renal pelvis, ureter or bladder & squamous epithelial cells arise from outer urethra or skin surface. Increased renal tubular epithelial cells are seen in nephrotic syndrome and tubular degeneration.

Casts : They are solid and cylindrical structures formed by precipitation of debris in the renal tubules. Urinary casts are formed only in the distal convoluted tubule or the collecting duct. Hyaline casts are composed primarily of Tamm-Horsfall proteins. Hyaline casts are seen in healthy individuals. RBC casts are formed when RBCs stick together and are seen in glomerular disease. WBC casts are seen in acute pyelonephritis and glomerulonephritis & denote inflammation of the kidneys. Granular and waxy casts are derived from renal tubular cell casts and are seen in Nephrotic Syndrome and tubular damage.

Crystals : Common crystals seen even in healthy individuals are calcium oxalate, triple phosphate crystals and amorphous phosphates. They may also be seen in renal stones. Abnormal crystals seen are cystine crystals in congenital cystinuria, tyrosine crystals in congenital tyrosinosis and leucine crystals in severe liver disease or maple syrup urine disease.

Bacteria : Diagnosis is established with urine culture. A colony count of >105/ml of one organism signifies significant bacteruria. Multiple organisms reflect contamination.

Yeast : It may be a contaminant. Budding yeast suggest a true candidal infection.